Back-illuminated global-shutter image sensor

ABSTRACT

A global shutter image sensor of a back-illuminated type includes a semiconductor substrate and pixels. Each pixel includes a photosensitive area, a storage area, a readout area and areas for transferring charges between these different areas. The image sensor includes, for each pixel, a protector extending at least partly into the substrate from the back of the substrate to ensure that the storage area is protected against back illumination.

FIELD OF THE INVENTION

The present invention relate to image sensors, and in particular, to aglobal-shutter image sensor.

BACKGROUND

In a global-shutter image sensor all of the pixels are exposedsimultaneously. This enables the capture of clearer images, notably whencapturing moving objects.

A global-shutter sensor conventionally comprises a set of pixels, witheach pixel comprising a photosensitive area, a storage area and areadout area. When the sensor is illuminated, electrons are generatedand accumulate in the photosensitive area. A control circuit allows thesimultaneous passage of the electrons from the photosensitive area tothe storage area, then to the readout area. A non-limiting example ofsuch a sensor and of its operation is described in published Frenchpatent application FR3000606.

Front-illuminated global-shutter image sensors exist but are difficultto integrate in three-dimensional integrated structures, notably due tothe necessity to make the storage area opaque to minimize or reducenoise.

SUMMARY

According to one embodiment, a back-illuminated global-shutter imagesensor integrated in a three-dimensional integrated structure isprovided.

The integrated image sensor may be adapted to a control mode known as aglobal shutter, and comprises a semiconductor substrate and a pluralityof pixels. Each pixel may comprise a photosensitive area, a storagearea, a readout area and areas for transferring charges between thesedifferent areas.

The sensor may be of the back-illuminated type and may further comprise,for each pixel, protection means or a protector extending at leastpartly into the substrate from its back and configured to ensure thatthe storage area is protected against back illumination.

The protectors may take different forms. In certain cases they can bepassive or active in the sense that they then carry out a transfer gatefunction. Furthermore, the protectors may also serve as light guides,notably for infrared radiation.

According to a first variation in which the protectors are passive, thestorage area may be at least partially delimited by at least one firstand second insulated vertical electrodes which extend from the front ofthe substrate. The first electrode may include a space corresponding toa first area for transferring charges between the photosensitive areaand the storage area.

In this first variation, the protectors may, for example, comprise asingle capacitive insulation trench which extends from the back into theelongation of the at least first and second insulated verticalelectrodes to cover the storage area.

According to another variation, the protectors may comprise at least onefirst and second capacitive insulation trenches which extend into thesubstrate from its back, respectively opposite at least the first andsecond vertical electrodes. A screen may extend on the back of thesubstrate between the two trenches to cover the storage area.

To ensure a better optical insulation of the storage area, theprotectors may non-electrically touch the first and the second insulatedvertical electrodes. This notably makes it possible to limit stillfurther the optical crosstalk phenomena familiar to the person skilledin the art.

It is also possible to leave a space between the protectors and thefirst and second insulated vertical electrodes. For example, the spacemay be on the order of 0.4 micrometers. According to another variation,part of the protectors may be active, i.e., each protector may compriseinsulated vertical electrodes with an area for transferring charges.

In other words, the protectors ensure, in this case, on the one hand afunction for protecting the storage area against the illumination fromthe back, and on the other hand, in the vicinity of the front, afunction of insulated vertical electrodes which, with a suitablepolarization, will ensure a charge-transfer function.

More specifically, according to one embodiment, the protectors maycomprise at least one first and second capacitive insulation trenchesextending into the substrate from its back and at least partiallydelimiting the storage area and forming, in the storage area, first andsecond insulated vertical electrodes. The first insulated verticalelectrode may include a space corresponding to a first area fortransferring charges between the photosensitive area and the storagearea.

The protectors may also comprise a protection screen extending on theback of the substrate between the at least one first and secondcapacitive insulation trenches to cover the storage area. The capacitiveinsulation trenches may each extend up to the front of the substrate.This notably makes it possible, during a step for producing thetrenches, to use, for example, the PMD (Pre-Metal Dielectric) dielectriclayer of the BEOL portion located on the front of the substrate as anetch-stop layer.

The capacitive insulation trenches may also extend up to a localizedimplanted area located in the vicinity of the front of the substrate.This makes it possible to leave a space between the end of the trenchesand the front of the substrate for potential implementation ofcomponents under the capacitive insulation trenches.

The protectors may comprise metal. Thus, the storage area may also beprotected against infrared light rays. The insulation trenches maydelimit the storage areas of two adjacent pixels while forming awaveguide for infrared rays reaching the photosensitive area of thepixel.

The image sensor may further comprise a control circuit adapted forapplying control signals to the insulated vertical electrodes. Thus, bypolarizing the electrodes to an adapted potential, it is possible tomake the electrons migrate from one region to another.

According to another aspect, a three-dimensional integrated structure isprovided. The structure may comprise a first electronic chip comprisingan image sensor as described above, and a second electronic chip. Thesecond electronic chip may comprise at least part of the controlcircuit. The two chips may be connected to one another by hybrid bondingof the metal-metal and insulator-insulator type.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features of the invention will become apparent uponexamining the detailed description of embodiments, which are in no waylimiting, and the appended drawings, in which:

FIGS. 1 to 6 illustrate embodiments of the invention.

DETAILED DESCRIPTION

FIG. 1 is a cross-sectional representation of a global-illuminationimage sensor according to one embodiment. The image sensor CAP comprisesa substrate 1 comprising a semiconductive region 100 with weak p-typedoping.

The front FS of the substrate is surmounted (i.e., placed on top) by aninterconnect region 2, commonly known to the person skilled in the artby the acronym BEOL (Back End Of Line). The interconnect region 2 isshown in part for the sake of simplification.

The substrate 1 furthermore comprises, on its back BS, ananti-reflective layer 3, surmounted by a layer 4 acting, as will be seenbelow, as a hardmask for the production of capacitive insulationtrenches. The layer 4, for example, may be silicon nitride.

A pixel matrix has been produced in the substrate. Although the sensorcomprises numerous pixels, only two identical and juxtaposed pixelsP_(i) and P_(i+1) have been shown here for the sake of simplification.

Each pixel comprises a photosensitive area 5 comprising the stack of thesemiconductive region 100, a first region 6 with n-type doping and alayer 9 with p⁺-type doping, extending from the front of the substrate1, forming a PNP photodiode. A storage area 8 is for storing charges,which is formed by the stack of the semiconductive region 100, of asecond region 7 with n-type doping and of the layer 9. Each pixel alsocomprises a readout area 26, not shown in the sectional view of FIG. 1but shown diagrammatically in FIG. 2.

FIG. 2 is a highly diagrammatic top view of an exemplary layout for aphotodiode and a storage area. It should be noted that this is only anexample and that any other layout is conceivable. A first spacecorresponding to a first transfer gate 11 is located between thephotodiode and the storage area. A second space corresponding to asecond transfer gate 23 is located between the storage area and thereadout area.

The storage area 8 is delimited by a first insulated vertical electrode10, a second insulated vertical electrode 12 and a third insulatedvertical electrode 25 (not shown in FIG. 1 for the sake ofsimplification). Each of the electrodes 10, 12 and 25 extend into thesubstrate from its front.

In this example, the depths of the electrodes are greater than thedepths of the first and second implanted regions 6 and 7. For example,the electrodes extend into the substrate to a depth of 4 micrometers.The photodiode 5 (photosensitive area) is, in this example, notablydelimited by the first electrode 10 of the storage area of the pixelP_(i+1), and by the second electrode 12 of the pixel P_(i).

The first electrode 10 includes the space 11 which divides the firstelectrode 10 into two portions and which forms the first transfer gate.The third electrode 25 includes the space 23 which divides the thirdelectrode 25 into two portions and which forms the second transfer gate.

The operation of such a sensor is briefly provided below. Initially, thesubstrate is fixed to a reference potential, for example, to ground, andthe three electrodes 10, 12 and 25 to a potential below the referencepotential. This causes an accumulation of holes along the walls of theelectrodes, and thereby creates a potential barrier in the transfer area11. The barrier blocks the exchanges of electrons between thephotosensitive area 5 and the storage area 8. The barrier also creates apotential barrier in the transfer area 23 by blocking the exchanges ofelectrons between the storage area and the readout area. When thephotodiode is illuminated, electron-hole pairs are photogenerated in thephotodiode and the electrons are accumulated in the region 6.

Next, during a first transfer phase, the two portions of the firstelectrode 10 are polarized to the same first potential above thereference potential, for example, a potential of 2 volts. This causesthe transfer of all of the accumulated electrons from the region 6 tothe region 7 of the storage area 8, via the transfer gate 11. Once thetransfer has been made, the first electrode 10 is once again polarizedto the reference potential to keep the electrons in the region 7 of thestorage area 8.

Then, during a second transfer phase, a second potential above thereference potential, for example, a potential of 2 volts, is applied tothe two portions of the third electrode 25. This causes the transfer,via the transfer gate 23, of all of the stored electrons from the region7 to the readout area 26. Here the value of the pixel will be read outby a readout circuit, which is not shown in the diagram for the sake ofsimplification.

In FIG. 1, the image sensor CAP is back illuminated. When the sensor isilluminated, the luminous flux FL therefore reaches the back BS of thesubstrate. Each storage area 8 should therefore be protected from thelight to avoid unwanted signals as much as possible. Thus, in thisexample, each storage area 8 is protected from the light by protectionmeans or protector MP.

In this embodiment, the protector MP for the pixel P_(i) comprise asingle capacitive insulation trench 13 which extends from the back ofthe substrate opposite the three electrodes 10, 12 and 25. This is tocover the storage areas and partially delimit the photodiodes 5 of thepixels P_(i) and P_(i+1).

In this example, the walls of the capacitive insulation trenches 13 arecovered by an insulating material 14, for example, silicon oxide. Thetrenches are filled with a metallic material 15, for example, copper.

It should be noted that not only does the capacitive insulation trench13 totally cover the storage area 8, but it additionally touches thethree electrodes 10, 12 and 25, yet without being in electrical contacttherewith. This contact is made by the insulating material 14 of thecapacitive insulation trench.

However, to avoid any risk of electrical contact during the productionof the trenches, provision can be made to leave a space between thecapacitive insulation trench 13 and the electrodes 10, 12 and 25. Forexample, a space of 0.4 micrometer may be left. The trench can, forexample, extend into the substrate to a depth of 2 to 16 micrometers.

The capacitive insulation trench 13 is adapted to be polarized to athird potential below the reference potential, for example, −3 volts.This is notably to limit the dark current. The capacitive insulationtrenches of the image sensor are passive, i.e., they do not act astransfer electrodes. However, they do form waveguides for infrared rays.

FIG. 3 illustrates an embodiment in which the protector MP for the pixelPi comprises a first capacitive trench 16 extending from the back of thesubstrate 1 opposite the first electrode 10, a second capacitiveinsulation trench 17 extending from the back of the substrate 1 oppositethe second electrode 12, and a third capacitive insulation trench, notshown in FIG. 3, extending from the back of the substrate 1 opposite thethird insulated vertical electrode 25.

Each capacitive insulation trench extends into the substrate over alength of 2 to 16 micrometers and is in mechanical contact, withoutbeing in electrical contact, with the electrode facing it. Eachcapacitive insulation trench also comprises an insulating layer 14 ofsilicon oxide and a copper filling 15. Once again, it would be possibleto leave a space on the order of 0.4 micrometers between each capacitiveinsulation trench and the electrode facing it.

The protector furthermore comprises an opaque screen 18, for example,metallic, which extends from the back of the substrate 1 to cover thestorage area 8 completely. The screen 18 is in contact with the twocapacitive insulation trenches 16 and 17, and with the third capacitiveinsulation trench.

The insulation trenches 16 and 17, the third insulation trench and theopaque screen may be produced during the same process step, for example,a process known as dual-damascene that is familiar to the person skilledin the art, and therefore comprises the same material. In this example,the trenches 16 and 17, as well as the opaque screen 18, comprise thesame metal 15, such as copper, for example.

Once again, the capacitive insulation trenches 16 and 17 and the thirdcapacitive insulation trench are adapted to be polarized to a potentialbelow the reference potential, for example, −3 volts. This is notably inorder to limit the dark current. Once again, the capacitive insulationtrenches 16 and 17 form waveguides, notably for infrared rays.

FIG. 4 illustrates an embodiment in which the protector MP comprises theinsulated vertical electrodes. In this embodiment, the protector MP forthe pixel P_(i) comprises a first capacitive trench 19, a secondcapacitive insulation trench 20, a third capacitive insulation trench,and an opaque protection screen 18.

The first capacitive trench 19 extends into the substrate from its backto its front. This is between the photosensitive area 5 and the storagearea 8.

The second capacitive insulation trench 20 extends into the substratefrom its back to its front. This is between the storage area 8 and thephotosensitive area of the neighboring pixel.

The third capacitive insulation trench, not shown in FIG. 5, extendsinto the substrate from its back to its front, between the storage area8 and the readout area 26. The three capacitive insulation trenchesthereby delimit the storage area 8.

The opaque protection screen 18 extends on the back BS of the substratebetween the two capacitive insulation trenches 19 and 20 to cover thestorage area 8. The first capacitive insulation trench 19 includes aspace in its lower part. This corresponds to the gate for transferringcharges 11 between the photosensitive area 5 and the storage area 8.

The third capacitive insulation trench (not shown) also includes a spacein its lower part. This corresponds to the gate for transferring charges23 between the storage area 8 and the readout area 26. The firstcapacitive trench 19 and the third capacitive trench are active, i.e.,they act as charge-transfer gates in their lower parts. The secondcapacitive insulation trench 20 is passive here.

According to another embodiment, illustrated in FIG. 5, the front of thesubstrate furthermore comprises localized implanted areas 24 located onboth sides of each storage area 8. The capacitive insulation trencheseach extend up to an implanted area 24. These implanted areas 24 canaccommodate electronic components, for example, transistors.

To produce a sensor as described above, a semiconductor substrate istaken as the starting point. On its front the various implanted areas ofthe various pixels are produced, as well as the electrodes 10, 12 and 25(FIGS. 1 and 3). Then the interconnect portion (BEOL) on the front ofthe substrate is produced.

Next, the substrate is turned over and the substrate is thinned from itsback. Next, the protectors MP are produced by etching and filling withmetal, according to one of the embodiments described previously andillustrated in FIGS. 1 to 5. In the case of FIGS. 4 and 5, thecapacitive insulation trenches of the protectors MP also form thevertical electrodes.

FIG. 6 illustrates an embodiment in which an image sensor CAP accordingto the embodiment described previously and illustrated in FIG. 1 hasbeen integrated in a three-dimensional integrated structure STR. Theintegrated structure STR comprises a first chip P1 comprising the sensorCAP, and a second chip P2 comprising a control logic circuit, forexample.

The first chip P1 comprises a plurality of pixels P_(i), P_(i+1),P_(i+2), etc. Each pixel comprises a photosensitive area 5 and a memoryarea 8 protected by the single capacitive insulation trench 13. Theinterconnect portion of the first chip P1 comprises first bonding padsBP1 which comprise copper and which extend from its front.

The second chip P2 comprises a semiconductor substrate 21, surmounted byan interconnect region 22. The interconnect region 22 of the second chipP2 comprises metal levels and vias which between them connect thevarious electronic components of the second chip P2, as well as bondingpads BP2 comprising copper located on its front.

The two chips P1 and P2 are joined to one another by hybrid bonding. Thehybrid bonding may be metal-metal and insulator-insulator. The controlcircuit CC intended to control the sensor and notably to polarize thevarious electrodes is located in the substrate of the chip P2.

It would also be possible for the control circuit CC to be locatedpartly in the chip P2 and partly in the chip P1, or even entirely in thechip P1. Similarly, the chip P2 could be a simple support, known to theperson skilled in the art by the term interposer.

Although a three-dimensional integrated structure STR comprising animage sensor according to the embodiment described in FIG. 1 has beendescribed, such a structure is compatible with the other embodimentsdescribed previously.

Furthermore, although capacitive insulation trenches comprising copperhave been described, the embodiments described previously are compatiblewith any type of metal, notably tungsten. Tungsten can advantageously beused for filling trenches with a very large aspect ratio (i.e., theratio of height to width).

That which is claimed is:
 1. A back-illuminated image sensor comprising:a semiconductor substrate; and a plurality of pixels in thesemiconductor substrate, each pixel comprising a photosensitive area, astorage area, a readout area, areas for transferring charges between thephotosensitive, storage and readout areas, and a protector extending atleast partly into the semiconductor substrate from a back of thesemiconductor substrate, the protector configured to protect the storagearea against back illumination, wherein each pixel further comprisesspaced apart first and second insulated vertical electrodes which extendfrom a front of the semiconductor substrate to at least partiallydelimit the storage area, the first insulated vertical electrodeincluding a space corresponding to a first area for transferring chargesbetween the photosensitive area and the storage area, and wherein theprotector comprises: spaced apart first and second capacitive insulationtrenches which extend into the semiconductor substrate from a back ofthe semiconductor substrate, respectively opposite the first and secondinsulated vertical electrodes, and a screen extending on the back of thesemiconductor substrate between the spaced apart first and secondcapacitive insulation trenches to cover the storage area.
 2. Theback-illuminated image sensor according to claim 1, wherein theprotector non-electrically contacts the first and second insulatedvertical electrodes.
 3. The back-illuminated image sensor according toclaim 1, wherein the protector comprises metal.
 4. The back-illuminatedimage sensor according to claim 1, further comprising a control circuitconfigured to apply control signals to the first and second insulatedvertical electrodes.
 5. The back-illuminated image sensor according toclaim 1, wherein the semiconductor substrate and the plurality of pixelsin the semiconductor substrate are configured so that theback-illuminated image sensor is a back-illuminated global shutter imagesensor.
 6. The back-illuminated image sensor according to claim 1,wherein the first and second insulated vertical electrodes are in directcontact with a portion of the storage area.
 7. The back-illuminatedimage sensor according to claim 1, wherein the semiconductor substratecomprises an anti-reflective layer, and wherein the protector extends atleast partly into the semiconductor substrate from a back of thesemiconductor substrate—and through the anti-reflective layer.
 8. Aback-illuminated image sensor comprising: a semiconductor substrate; anda plurality of pixels in the semiconductor substrate, each pixelcomprising a photosensitive area, a storage area, a readout area, areasfor transferring charges between the photosensitive, storage and readoutareas, and a protector extending at least partly into the semiconductorsubstrate from a back of the semiconductor substrate and configured toprotect the storage area against back illumination, wherein theprotector comprises: spaced apart first and second capacitive insulationtrenches extending into the semiconductor substrate from a back of thesemiconductor substrate to at least partially delimit the storage areaand forming, in the storage area, spaced apart first and secondinsulated vertical electrodes, the first insulated vertical electrodeincluding a space corresponding to a first area for transferring chargesbetween the photosensitive area and the storage area; and the protectorcomprising a screen extending on the back of the semiconductor substratebetween the first and second capacitive insulation trenches to cover thestorage area.
 9. The back-illuminated image sensor according to claim 8,wherein the first and second capacitive insulation trenches extend fromthe back of the semiconductor substrate to a front of the semiconductorsubstrate.
 10. The back-illuminated image sensor according to claim 8,wherein the semiconductor substrate comprises on a front thereof, foreach pixel, spaced apart implanted areas; and wherein the first andsecond capacitive insulation trenches extend from the back of thesemiconductor substrate to the spaced apart implanted areas.
 11. Theback-illuminated image sensor according to claim 8, wherein thesemiconductor substrate comprises an anti-reflective layer, and whereinthe protector extends at least partly into the semiconductor substratefrom a back of the semiconductor substrate—and through theanti-reflective layer.
 12. A three-dimensional integrated structurecomprising: a first electronic chip comprising a back-illuminated globalshutter image sensor comprising a semiconductor substrate, a pluralityof pixels in the semiconductor substrate, each pixel comprising aphotosensitive area, a storage area, a readout area, areas fortransferring charges between the photosensitive, storage and readoutareas, and a protector extending at least partly into the semiconductorsubstrate from a back of the semiconductor substrate, the protectorconfigured to protect the storage area against back illumination; and asecond electronic chip comprising a control circuit for applying controlsignals to the back-illuminated global shutter image sensor, wherein thefirst and second electronic chips are joined together, wherein eachpixel further comprises spaced apart first and second insulated verticalelectrodes which extend from a front of the semiconductor substrate toat least partially delimit the storage area, the insulated verticalfirst electrode including a space corresponding to a first area fortransferring charges between the photosensitive area and the storagearea, and wherein the protector comprises: spaced apart first and secondcapacitive insulation trenches which extend into the semiconductorsubstrate from a back of the semiconductor substrate, respectivelyopposite the first and second insulated vertical electrodes, and ascreen extending on the back of the semiconductor substrate between thespaced apart first and second capacitive insulation trenches to coverthe storage area.
 13. The three-dimensional integrated structureaccording to claim 12, wherein the first and second electronic chips arejoined together via hybrid bonding, with the hybrid bonding comprisingat least one of metal-metal and insulator-insulator type bonding. 14.The three-dimensional integrated structure according to claim 12,wherein the protector non-electrically contacts the first and secondinsulated vertical electrodes.
 15. The three-dimensional integratedstructure according to claim 12, wherein the semiconductor substratecomprises an anti-reflective layer, and wherein the protector extends atleast partly into the semiconductor substrate from a back of thesemiconductor substrate—and through the anti-reflective layer.
 16. Athree-dimensional integrated structure comprising: a first electronicchip comprising a back-illuminated global shutter image sensorcomprising a semiconductor substrate, a plurality of pixels in thesemiconductor substrate, each pixel comprising a photosensitive area, astorage area, a readout area, areas for transferring charges between thephotosensitive, storage and readout areas, and a protector extending atleast partly into the semiconductor substrate from a back of thesemiconductor substrate, the protector configured to protect the storagearea against back illumination; and a second electronic chip comprisinga control circuit for applying control signals to the back-illuminatedglobal shutter image sensor, wherein the first and second electronicchips are joined together, wherein the protector comprises: spaced apartfirst and second capacitive insulation trenches extending into thesemiconductor substrate from a back of the semiconductor substrate to atleast partially delimit the storage area and forming, in the storagearea, spaced apart first and second insulated vertical electrodes, thefirst insulated vertical electrode including a space corresponding to afirst area for transferring charges between the photosensitive area andthe storage area, and the protector comprising a screen extending on theback of the semiconductor substrate between the first and secondcapacitive insulation trenches to cover the storage area.
 17. Thethree-dimensional integrated structure according to claim 16, whereinthe first and second capacitive insulation trenches extend from the backof the semiconductor substrate to a front of the semiconductorsubstrate.
 18. The three-dimensional integrated structure according toclaim 16, wherein the semiconductor substrate comprises on a frontthereof, for each pixel, spaced apart implanted areas; and wherein thefirst and second capacitive insulation trenches extend from the back ofthe semiconductor substrate to the spaced apart implanted areas.
 19. Thethree-dimensional integrated structure according to claim 16, whereinthe semiconductor substrate comprises an anti-reflective layer, andwherein the protector extends at least partly into the semiconductorsubstrate from a back of the semiconductor substrate—and through theanti-reflective layer.
 20. A method for protecting global shutter imagesensor from back-illumination, the global shutter image sensorcomprising a semiconductor substrate, a plurality of pixels in thesemiconductor substrate, with each pixel comprising a photosensitivearea, a storage area, a readout area, and areas for transferring chargesbetween the photosensitive, storage and readout areas, the methodcomprising: forming a protector that extends at least partly into thesemiconductor substrate from a back of the semiconductor substrate toprotect the storage area against back illumination, wherein each pixelfurther comprises spaced apart first and second insulated verticalelectrodes which extend from a front of the semiconductor substrate toat least partially delimit the storage area, the first insulatedvertical electrode including a space corresponding to a first area fortransferring charges between the photosensitive area and the storagearea, and wherein forming the protector comprises: forming spaced apartfirst and second capacitive insulation trenches which extend into thesemiconductor substrate from a back of the semiconductor substrate,respectively opposite the first and second insulated verticalelectrodes, and forming a screen extending on the back of thesemiconductor substrate between the spaced apart first and secondcapacitive insulation trenches to cover the storage area.
 21. The methodaccording to claim 20, wherein the protector non-electrically contactsthe first and second insulated vertical electrodes.
 22. The methodaccording to claim 20, wherein the semiconductor substrate comprises ananti-reflective layer, and wherein the protector extends at least partlyinto the semiconductor substrate from a back of the semiconductorsubstrate—and through the anti-reflective layer.
 23. A method forprotecting global shutter image sensor from back-illumination, theglobal shutter image sensor comprising a semiconductor substrate, aplurality of pixels in the semiconductor substrate, with each pixelcomprising a photosensitive area, a storage area, a readout area, andareas for transferring charges between the photosensitive, storage andreadout areas, the method comprising: forming a protector that extendsat least partly into the semiconductor substrate from a back of thesemiconductor substrate to protect the storage area against backillumination, wherein forming the protector comprises: forming spacedapart first and second capacitive insulation trenches extending into thesemiconductor substrate from a back of the semiconductor substrate to atleast partially delimit the storage area and forming, in the storagearea, spaced apart first and second insulated vertical electrodes, thefirst insulated vertical electrode including a space corresponding to afirst area for transferring charges between the photosensitive area andthe storage area; and forming a screen extending on the back of thesemiconductor substrate between the first and second capacitiveinsulation trenches to cover the storage area.
 24. The method accordingto claim 23, wherein the first and second capacitive insulation trenchesextend from the back of the semiconductor substrate to a front of thesemiconductor substrate.
 25. The method according to claim 23, whereinthe semiconductor substrate comprises an anti-reflective layer, andwherein the protector extends at least partly into the semiconductorsubstrate from a back of the semiconductor substrate—and through theanti-reflective layer.